<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .red {
            color: red;
        }

        .size {
            font-size: 30px;
        }
    </style>
</head>
<body>
<!--一个标签要使用多个类名需要用空格隔开-->
<p class="red size">111</p>
<p class="red">222</p>
<div class="red">这个div也要变成红色</div>
</body>
</html>